Biodegradable synthetic lubricants

ABSTRACT

The invention relates to synthetic esters made from materials which can be derived from renewable resources.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 60/291,816, filed May 17, 2001.

FIELD OF THE INVENTION

[0002] This invention is in the art of synthetic lubricants based on esters.

BACKGROUND OF THE INVENTION

[0003] There is a concern over the biodegradability and other eco-toxicity properties of lubricants used in environmentally sensitive applications such as forestry, mining, and agriculture. Equipment manufacturers who use these lubricants have observed that product performance declines when so-called “biodegradable” based fluids are used. In addition to the environmentally friendly qualities such as biodegradability, recent regulatory initiatives are directed to using fluids derived from renewable resources (e.g., from agricultural products).

[0004] TMP oleate (trimethylolpropane trioleate) is considered the industry standard in formulating biodegradable hydraulic fluids. It has the appropriate viscosity (circa 46 cst at 40° C.) and biodegradability. However, TMP oleate has poor oxidative and thermal stability, and poor low temperature fluidity (e.g., pour point). An effective replacement for TMP oleate has heretofore been unavailable.

[0005] U.S. Pat. Nos. 5,658,863; 5,681,800; 5,728,658; 5,767,047; and 5,817,607 are directed to biodegradable lubricants comprising, inter alia, esters derived from branched acids. These acids are based on propylene oligomers, which are not considered renewable natural products.

[0006] Recently French Patent Application 2,763,597 (A1) described a lubricating oil composition for using industrial materials used in the preparation of products intended for living biological organisms, e.g., food preparation. The reference does not address lubricating oils having the appropriate properties to replace TMP oleate.

[0007] Thus there is a need for a fluid having the appropriate technical properties required by a lubricant while at the same time having environmentally friendly properties and being derived from renewable resources.

SUMMARY OF THE INVENTION

[0008] The present inventor has found that esters prepared by esterifying linear acids with alcohols, wherein both the acids and alcohols are derived from natural products, provide lubricants having good biodegradability and eco-toxicity requirements, as well as technical requirements that are similar to or even superior to lubricants currently used by equipment manufacturers. In particular, an ester made by reacting sorbitol with 3-6 molar equivalents of a mixture of at least two different fatty acids provides a lubricant equal to or superior to TMP oleate.

[0009] Sorbitol can be derived from natural products such as corn oil (hydrogenation of glucose) and certain linear fatty acids are derived from natural products such as coconut oil.

[0010] These and other objects, features, and advantages of the present invention will become apparent as reference is made to the following detailed description of the preferred embodiments, specific examples, and the attached claims.

DETAILED DESCRIPTION OF THE INVENTION

[0011] A polyol derived from natural sources, preferably sorbitol, is partially or fully esterified with at least two different linear acids derived from natural sources (e.g., renewable), preferably two or more different fatty acids, each independently having from 6 to 12 carbon atoms, still more preferably two different linear fatty acids selected from C6, C8, C10, and C12 fatty acids. It is preferred that sorbitol is esterified with 3-6 molar equivalents of the mixed fatty acids. As used herein the term “mixed” means at least two fatty acids having different carbon number. Three or more different fatty acids may also be used. Examples of suitable acids include, but are not limited to, caprylic acid, capric acid, and lauric acid.

EXAMPLE

[0012] A sorbitol ester fluid was made by refluxing 1.25 moles of sorbitol with 5.5 moles of a commercially available 60:40 wt % mixture of caprylic acid/capric acid (C8 and C10, respectively) at 240° C. The reaction proceeded until the hydroxyl number was less than 3. The hydroxyl number can be determined by one of skill in the art. Unreacted acids were removed by vacuum distillation. A further reduction in acidity was obtained by neutralizing the free acid with Ca(OH)2 until the total acid number (TAN), as determined by ASTM D974, was less than 0.1. The properties of this ester fluid are compared with TMP oleate in Table 1, below. TABLE 1 Comparison of Esters TMP oleate Sorbitol caprylate/caprate Viscosity at 100° C., cSt 9.4 7.5 Viscosity at 40° C., cSt 48.0 48.1 Pour point, ° C. −42 −48 Flash pt, ° C. >310 272 Biodegradability, % by OECD 81 83 method 301B

[0013] Viscosity was determined according to ASTM D445. Pour point is determined according to ASTM D97. Flash point is determined according to ASTM D92.

[0014] As shown in Table 1, the properties of the ester fluid according to the present invention are similar with respect to the technical aspects of the industry standard TMP oleate, but having an improved biodegradability and pour point.

[0015] An ISO VG 46 hydraulic oil (meaning that the oil has a viscosity of 46 centistokes, or “46 cSt” at 40° C.) was blended with a standard additive package (Rhein Chemie RC 9308) containing rust inhibitor, anti-oxidants, and an anti-wear additive, with the sorbitol ester fluid as the base stock. A comparative formulation was made in the same way but using TMP oleate as the base stock. A comparison of the two compositions is shown below in Table 2. TABLE 2 Comparison of Formulated Hydraulic Fluid Target TMP oleate Sorbitol caprylate/caprate Dry TOST, hours 500 minimum 332 1000+ Oxidation @ 175° C./72 hr % Viscosity increase 25 max. 219 5.9 TAN increase 2 max.  2 0.3 ASTM D665 rust A & B Pass Pass Pass Demulsibility, (ASTM D1401) 40 max.  20 15 minutes Foam by ASTM D892, mls/ 15/0 max. 10/0 10/0 mls

[0016] As shown in the table above, the stability of the oil based on the sorbitol ester fluid is superior to the oil based on the oleate ester. This is shown in the dry TOST test (Modified ASTM D943) and the oxidation and corrosion test. The TOST test is modified from the published method D943 in that no water is included in the test. The oxidation and corrosion test is according to FTM 5308, modified in that only copper, aluminum, and steel are used as metal specimens. The TOST life is reported in hours to achieve a TAN of greater than 2. The oil based on TMP oleate lasted 332 hours while the oil based on the sorbitol ester lasted more than 1000 hours.

[0017] In the oxidation and corrosion test, an oil sample is held for 72 hours at 175° C. in the presence of copper, steel, and aluminum specimen. Air is sparged through the sample at a rate of 5 liter/hr. The change in TAN and viscosity are reported at the completion of the test. The viscosity increase of the oleate ester was about 35 time higher than the sorbitol ester, and the TAN increase was about 4 times higher. These performance features are useful to prolong the service life of ester based hydraulic fluids.

[0018] The demulsibility test is run according to ASTM D1401. 40 mls of oil and 40 mls of water are mixed in a graduated cylinder and held at 180° F. The amount of time for the oil and water to separate is recorded. The sorbitol ester separated water faster than the TMP oleate, which is beneficial in wet hydraulic systems.

[0019] Foaming is measured by ASTM D892. Air is blown through a porous stone into a cylinder of oil. The volume of foam produced and the time required for the foam to collapse is recorded.

[0020] The present invention is directed to an hydraulic oil formulated using an ester lubricant based on polyols and fatty acids derived from natural products, preferably an ester derived by esterifying sorbitol with a mixture of at least two linear fatty acids, even more preferably wherein the at least two linear acids are independently selected from acids having from 6 to 12 carbon atoms, and most preferably wherein the acids mixture comprises caprylic acid [CH3(CH2)6COOH] and capric acid [CH3(CH2)8COOH]. The polyol, most preferably sorbitol, can be esterified using at least 3 molar equivalents of total acids in the linear fatty acid mixture, preferably at least 4 molar equivalents of total acids in the linear fatty acid mixture, and more preferably at least 5 molar equivalents of total acids in the linear fatty acid mixture (equivalents based on the number of hydroxyl groups in the polyol). It can be esterified using less than 4 molar equivalents of mixed linear fatty acids (i.e., less than four alcohol groups esterified) or more than 4 molar equivalents, up to fully esterified. The hydraulic oil according to the present invention is preferably used in environmentally sensitive applications such as forestry, mining, and agriculture.

[0021] The invention has been described above with particular attention to the most preferred embodiments, but it will be appreciated by one of ordinary skill in the art in possession of the present disclosure that many variations and modifications of the aforementioned may be practiced within the spirit and scope of the appended claims. Those particular preferred embodiments include:

[0022] A lubricant formulation having an ester as the base material, the improvement comprising making said ester by reacting sorbitol with 3 to 6 molar equivalents of mixed linear fatty acids; or more preferably wherein said mixed linear fatty acids are selected from C6 to C12 acids; or even more preferably wherein said mixed linear fatty acids are selected from C6, C8, C10, and C12 acids; or still more preferably wherein said mixed linear fatty acids are caprylic and capric acids; yet still more preferably wherein said base material is made by reacting sorbitol with a 60:40 wt % mixture of caprylic and capric acids.

[0023] In another preferred embodiment, the present invention is directed to:

[0024] An hydraulic fluid for equipment used in forestry, mining, or agricultural operations comprising a base stock consisting essentially of an ester derived from natural products; more preferably wherein said ester is made esterifying at least one linear acid derived from corn with linear alcohols derived from coconut oil. 

I claim:
 1. A lubricant formulation having an ester as the base material, the improvement comprising making said ester by reacting sorbitol with 3 to 6 molar equivalents of mixed linear fatty acids.
 2. The lubricant formulation according to claim 1, wherein said mixed linear fatty acids are selected from C₆ to C₁₂ acids.
 3. The lubricant formulation according to claim 1, wherein said mixed linear fatty acids are selected from C₆, C₈, C₁₀, and C₁₂ acids.
 4. The lubricant formulation according to claim 1, wherein said mixed linear fatty acids are caprylic and capric acids.
 5. The lubricant formulation according to claim 1, wherein said base material is made by reacting sorbitol with a 60:40 wt % mixture of caprylic and capric acids.
 6. A hydraulic fluid for equipment used in forestry, mining, or agricultural operations comprising a base stock consisting essentially of an ester derived from natural products.
 7. The hydraulic fluid according to claim 6, wherein said ester is made esterifying at least one linear acid derived from coconut oil with a linear alcohol derived from corn. 